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Astron. Astrophys. 328, 247-252 (1997)

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2. The formation of a hole in the inner disk region

A number of other processes besides disk evaporation might also lead to hole formation. The desirability of hole formation had already been pointed out in Meyer & Meyer-Hofmeister (1989) where it was suggested that remaining disk magnetic field might lead to a lowering of the surface density in the inner disk region below the lower critical value for the existence of bistable disk structure. This would prevent a heating wave coming from the outside to transform the inner disk to a hot UV radiating state and is equivalent to hole formation. Later Livio & Pringle (1992) discussed as alternative to the evaporation mechanism the possibility that white dwarf magnetic fields, too weak to be observable, in other ways might sweep the interior disk matter away in quiescence. Speculative reasoning on the dwarf nova oscillation phenomenon (Meyer 1997) lets us appear the assumption of such field less attractive though does not exclude it. Another possibility to deplete an inner disk region in quiescence is to irradiate the disk surface from the still hot white dwarf and thereby raise the disk temperature which would result in mass flow towards the white dwarf (King 1997). We point out that the coronal evaporation process itself deposits heat into the underlying cool disk, possibly more important than irradiation from the white dwarf which also accelerates mass depletion during the early phase of hole formation. All such models still require coronal temperatures near the white dwarf and presumably a thermal boundary layer as discussed in Liu et al. (1995) in order to account for the observed X-rays in quiescence.

We use the evaporation model in its simple form here because it allows clear qualitative predictions in reasonable agreements with observation (Meyer & Meyer-Hofmeister 1994).

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998